1. Field of the Invention
The present invention generally relates to an ultrasonic diagnostic apparatus for acquiring ultrasonic images of a biological body by applying delay times to energizing signals for a plurality of transducer elements arranged in an array form, and also to echo signals reflected from interiors of the biological body. More specifically, the present Invention is directed to an ultrasonic diagnostic apparatus capable of improving image qualities of the acquired ultrasonic images with respect to unequal portions within the biological body based upon fluctuations in arrival times of the echo signals.
2. Description of the Prior Art
In a typical ultrasonic diagnostic apparatus, a plurality of transducer elements are arranged in an array form. Data on delay times which are determined based on geometrical positional information with respect to the respective transducer elements, are electrically given to both the energizing signals and echo signals reflected from interiors of a biological body. The energizing signals are used to transmit ultrasonic pulses from the transducer elements toward the biological body. Based upon the delay time data, a focal point of the, echo signals is defined by the known in-phase additional process so as to obtain a desirable ultrasonic image of the biological body. The focal point is scanned under electronic scanning control, for instance, In a plane form. More specifically, while several tens to several hundreds of rasters are transmitted/receiver, a "B-mode" ultrasonic image is obtained in an image constructive unit.
The above-described known ultrasonic imaging technique will now be described more in detail with reference to FIGS. 1 to 3.
As shown in FIG. 1, under such conditions that the velocity of sound which passes through a propagation medium I.sub.o from a transducer element "V" to a focal point is uniform, and this sound velocity is known, when the respective transducer elements are energized by receiving data on delay times "d.sub.1 " to "d.sub.n " ("n" being an integer greater than 2) which has been defined based on the focal point "C" and the geometric positional data, a focal point "C" at which ultrasonic pulses are fully focused can be formed without serious phase shifts.
However, in case that organs within a biological body "P" are to be ultrasonic-imaged, as represented in FIG. 2, the ultrasonic pulses transmitted from the transducer elements "V" are propagated through a surface layer "P.sub.I " biological body "P", which is constructed of fat and muscle made of unequal propagation mediums I and II. If the delay times "d.sub.1 " to "d.sub.n " are given to process the energizing signals for the transducer elements "V" is a similar method as in FIG. 1, there are phase shifts in the ultrasonic pulses near another focal point C'. As a result, this focal point C' is widened, as compared with the first-mentioned focal point C, and also both special resolution and contrast resolution of the resultant ultrasonic image are lowered.
Also to form such a sufficiently focused point "C", even when the organs having the unequal propagation mediums I and II are ultrasonic-imaged without any phase shift, it has been proposed that the delay correction values which have been obtained by way of the conventional correlation method, are added to the delay times determined by the geometric positional information on the respective transducer elements and this focal point C. Thus, the image characteristics of the ultrasonic images are improved. This conventional correlation method is described in, for example, Japanese Patent Disclosure 53-51846 (1988).
In accordance with one of the conventional correlation, as illustrated in FIG. 3, after the correlation function "F" has been calculated with respect to one pair of echo signals received by two certain transducer elements (see FIG. 2), time "t.sub.p " indicative peak value "F.sub.p " in this correlation function "F" is found out; a time difference ".DELTA.t" between this time "t.sub.p " and a predetermined reference time "t.sub.o " is calculated; and then the delay times produced in the echo signals received by two certain transducer elements are corrected based on this time difference information ".DELTA.t".
However, since the above-described conventional correlation method requires an extremely large computation scale, there are first problems that this correlation method is not suitable for improving the realtime image processing and impedes a compactness of the ultrasonic diagnostic apparatus.
On the other hand, it is impossible to consider another delay correction method with respect to plural focal points.
As shown in FIG. 4, when ultrasonic pulse beams are focused onto two focal points "0" and "Q" of the biological body "P" which are positioned along a depth direction of this body by employing an ultrasonic probe "P.sub.b " having "M" pieces of transducer elements, tile propagation paths these ultrasonic pulse beams are different from each other. In such a case, delay correction values different from each other are required with respect to two focal points "0" and "Q" by measuring the phase differences (i.e., time difference in propagation) of the echo signals.
In actual, not only two focal points "0", "Q", but also many other focal points are present within the biological body "P". Assuming now that a total number of ultrasonic scanning beams is "i" and a total number of focal points along each of the ultrasonic scanning beams is "j", a total focal point of this biological body "P" to be imaged amounts to 1.times.J. Normally, "1"=100 to 300 and "j"=10 to 30. Accordingly, the entire focal points amount to 1000 to 9000.
There is a second problem that a large quantity delay correction values must be calculated based on the conventional correlation method, which is not practically possible.
Furthermore, there are non-reflection (non-echo) portions within a biological body such as cholecystics and blood vessels, from which ultrasonic beams are not reflected as the echo pulses. If a focal point to ultrasonic-imaged is selected at cholecystics, no reflection signal (echo signal) is obtained therefrom, that no delay correction value of this focused cholecystics portion is calculated. Accordingly, there is a third problem that reflection signals suitable for ultrasonic measurements are not always obtained.